This invention relates to a system for authorizing a receiver to decode a scrambled signal in which an authorizing code is transmitted to said receiver. The invention also relates to a transmitter and a receiver for use with such a system. The invention in particular, though not exclusively, is concerned with authorizing television receivers to decode scrambled television signals.
For scrambled television transmissions, a number of proprosals have been made as to how subscribers may be authorized to view scrambled television signals on their television receivers. Such proposals have included the use of an active card which is introduced into the receiver together with a personal identification number (PIN) keyed in by the subscriber. One proposal that has been considered is that the authorization should be transmitted as a code from the television transmitter to the television receiver with each subscriber being allocated a unique authorization code, no action being required by the subscriber at the receiver. The time taken to sequentially transmit these individual authorization codes would be considerable. If it is assumed that a subscription television service has about 10,000,000 subscribers with each subscriber having an individual authorization code of (say) 50 bits transmitted at an effective data rate of 150 Kbits/second, then the time taken to sequentially transmit all the authorization codes would be on the order of one hour. This would mean that each subscriber wishing to receive a given (scrambled) program would have to have his television receiver energised, possibly in a stand-by condition, for at least an hour prior to the commencement of the programme which may not always be convenient. In addition, with such a long period taken to transmit all the authorization codes, it would not be possible to authorize the viewing of that program once transmission of that program had started.